#include <iostream>
#include <vector>

using namespace std;

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
};


void flatten(TreeNode* root) {

    if(root == nullptr) return;
    TreeNode* left = root->left;
    TreeNode* right = root->right;
    root->left = nullptr;
    root->right = left;
    flatten(left);
    flatten(right);
    TreeNode* p = root;
    while(p->right){
        p = p->right;
    }
    p->right = right;
}